Intraspecific variation in interspecific competitive ability: Not all competitors are created equally

Background/Question/Methods

Intraspecific variation in feeding traits can influence competitive ability and alter the outcome of competitive interactions.  We used an experimentally tractable competitive system to explore how intraspecific variation in resource acquisition and life-history traits can alter population dynamics and the outcome of interspecific competition.   We competed seven genotypes of the planktonic grazer Daphnia dentifera that were known to vary in a variety of competitive traits with a single genotype of Ceriodaphnia quadrangula. Each D. dentifera genotype was raised in separate microcosms both alone and in the presence of C. quadrangula. C. quandragula was also raised alone. The 15 treatments were fed daily for thirty days and population densities were tracked.  A two-way, nested ANOVA was performed on the D. dentifera integrated population densities to test for an overall effect of competition, genotype and their interaction.  A one-way ANOVA was performed on the C. quandrangula integrated population densities of the seven interspecific competition treatments to test for a genotypic effect of D. dentifera on the population growth of C. quadrangula.  Traits deemed important in competitive interactions were measured for the seven clones and statistical analyses were performed to find any correlations between certain traits of the clones and their competitive ability.

Results/Conclusions

Interspecific competition resulted in a significant decrease in the integrated population densities of all seven clones of D. dentifera.  Regardless of the presence of C. quandrangula, the seven different clones achieved significantly different population densities, revealing intraspecific variation in the intrinsic population growth strategies of D. dentifera.  Most interestingly, however, was the presence of a significant competition x genotype interaction among the D. dentifera clones, which indicates the presence of intraspecific variation in the competitive response of D. dentifera to C. quadrangula.  In parallel to this result, significant variation was found in the population densities achieved by the single genotype of C. quadrangula when competing with the seven D. dentifera clones.  Hence, we detected some sort of intraspecific variation among D. dentifera that the competitor was responding to.  Competitive traits including feeding rate, conversion efficiency and birth rate were found to significantly vary among the seven clones, however none were directly correlated with competitive ability.  In conclusion, we show that all genotypes of D. dentifera are not created equally in their ability to compete with C. quadrangula.  Incorporating intraspecific variation into studies of competition is crucial to understanding the mechanisms by which competition can alter ecological and evolutionary processes.